At the least, book six in the Harry Potter series does what it needs to: it moves the characters along in their personal and magical development; it fills in some previous gaps of knowledge; especially with regard to Voldemort's past; it continues to raise the level of urgency and danger as previous books have, it adds a few more mysteries/cliffhangers; and it ends with a clear pointing toward a final climactic battle. The problem, though, is that the book does the least one would expect and no more.
Placing it in the context of the earlier books, I'd argue that book three (Prisoner of Azkaban) is the strongest book so far. It's leaner than the last three, more focused, and has the greatest emotional impact. After Azkaban, I'd put book five (Order of the Phoenix) as the next strongest. Though a bit overlong, especially in the lengthy battle scene at the end, Phoenix shows a growth in character (especially in the admirably risky way that Rowling allows Harry to be an unlikable teenager) and a deeper emotional impact (especially when Harry learns more about his father as a youth) than the others. Book four (Goblet of Fire) suffers from being way too long and formulaic, while Book One, though much shorter, is simply average. Leaving Book Two (Chamber of Secrets) as the "middle quality" book–better than books one and four, not as strong as books three and five. Half-Blood Prince falls in along with Chamber of Secrets.
The book opens somewhat differently than the others, with a promising look at the Muggle side of things--specifically the interaction between the Muggle Prime Minister and his magical counterpart, who in recent years has had to keep the Muggle Prime Minister informed of what's been happening with the wizardly criminals, most recently the return of Voldemort (a known Muggle murderer). We move on from there to the more usual beginning--the impending return to Hogwarts (unfortunately, the Muggle connection is dropped completely).
As we've come to expect, Harry, Hermione, and Ron go through their typical preparations (getting books and robes etc), this time under heavy security as battle is being waged between the wizards and the Death Eaters (several wizards have already been killed, an event that occurs pretty regularly in the book; though mentioned mostly as asides there is a lot more death in this one than previous ones). They then go to school and attend their various classes, get in confrontations with Draco Malfoy and Professor Snape, play Quidditch, and do battle with Voldemort.
Without going into spoilers, the specifics involve Harry's absolute belief that Draco has been tasked with a specific job by Voldemort (and his running dispute with Hermione and Ron over that), Harry's absolute belief that Snape can't be trusted (and his running dispute with Dumbledore over that), Dumbledore's staggered revelations to Harry about Voldemort's past (beginning with Voldemort's parents), several attempts at murder within Hogwarts, and Dumbledore's increasingly active role in trying to weaken Voldemort and his increased willingness to let Harry take part. The sidestories involve Harry, Ron, and Hermione's growing romantic entanglements and, of course, Quidditch.
The plot never really reaches the point of being compelling. There is a lot of talk in the book, too much so, with Dumbledore especially doing a lot of explaining of things. By now the Quidditch material has mostly grown tiresome and while there is less of it here than in some other books, it mostly bogs the story down, even if only for a short while. The sections on Voldemort are interesting from an information standpoint though not particularly dramatic and the staggered means of conveying them to Harry (and thus to the reader) seems overly contrived. As does Dumbledore's need to "confirm" some things. It all seems too artificial a means to manipulate the reading experience. The scenes with Draco and Snape are nothing we haven't seen before. We know they're supposed to feel more intense since things are clearly coming to a head with Voldemort, but they are overly familiar and so robbed of any true sense of urgency and intensity with the exception of the final scene. Harry's discovery and use of a strange book with anonymous writing in it is also a bit too familiar from past books in the series. The romance adds a light touch and a sense of reality, but nothing unexpected. The book picks up in originality and intensity only toward the very end as Dumbledore and Harry begin a more active response to taking on Voldemort (though here as well some of the plot seems too contrived and the major revelation with regard to Voldemort not strikingly original).
As advertised, there is a major death in the book. Without giving a spoiler, I'll just say it's not all that unexpected and while the death sparks some obvious emotion, I'm not sure it really feels earned. It certainly doesn't have the emotional impact of Harry looking in the mirror at his folks or hearing his mother when the dementor's are around or seeing his father's patronus or seeing what happened between his father and Snape--all of which were more natural and more deeply felt.
The plot therefore is mostly serviceable, perhaps even disappointing. Harry's character continues to mature--one starts to see him truly coming into his own independence in fits and starts. The other characters don't really undergo much development beyond what we've seen. And one wishes Rowling would trust the characters' actions/descriptions to convey things without her having to clue us in by telling us someone was "raging" or someone said something "firmly" or "angrily".
The book gets us to the point it needs to--a more mature, more independent and confident and determined Harry bound to finish things one way or the other with Voldemort, joined by his equally determined friends. But it doesn't get us there with the spark of some of the earlier books or the originality or the emotional depth/impact of some of them. And for the first time one senses a writer behind the scenes manipulating things all too artificially. For those reasons Half-Blood Prince, while a decent book, doesn't rise to the level of Azkaban or Phoenix. Here's hoping the final one does.
(a review form Amazon) i agree more or less with this.